The present invention relates to a high pressure processing method and apparatus applicable to a development, cleaning, etch, rinse, displacement or drying process for a sample or processing object, such as semiconductor wafers or liquid-crystal substrates, having a super-microstructure.
As well known, in the field of semiconductor chip design, structural minimization is being carried out with great speed. While the wire size in a chip was about 1 μm decade ago, it is currently reduced down to about 0.18 μm. Further, a device with a wire size of 0.13 μm is close to practical use, and various technical developments have already started with a specific goal of 0.10 to 0.07 μm or stretched to 0.05 μm.
On the other hand, such progressive structural minimization involves some new technical problems which are unsolvable with conventional techniques. By way of example, one of currently concerned major technical problems is the collapse or breakdown of the microstructure due to capillary force. Specifically, most of semiconductor chips are produced by subjecting a silicon wafer to a number of wet processing, that is, processing using liquid. For example, in a photoresist development process, a liquid developer or alkaline aqueous solution is rinsed using pure water, and then dried. In the drying process, the wafer soaked with the water is exposed to the interface between liquid (pure water) and gas, and a high surface tension generated in the interface acts on adjacent wall-like developed photoresist layers, which stand on the silicon wafer substrate in parallel with each other, to pull them close to each other. Finally, the surface tension leads to the collapse phenomenon of the photoresist layers.
Such a phenomenon also occurs in manufacturing of electromechanical devices, so-called MEMS (Micro Electro Mechanical System), having a low-rigidity structure such as a micro-cantilever structure. In a typical production process of this type of devices, after removing a sacrifice layer on a substrate by an etch process using a hydrofluoric acid solution or the like, the substrate with cantilever structures is rinsed out using rinse liquid, and then dried. In this case, it is also known that a capillary force arising during evaporation of the rinse liquid causes undesirable sticking between the cantilever structures or between the cantilever structure and the substrate.
Among various techniques developed for solving the above problem, a drying technique using a high pressure fluid, particularly so-called “supercritical fluid”, is recently regarded as one of noteworthy technologies. This drying technique using the supercritical fluid allows a microstructure to be dried without formation of any gas-liquid interface or under the condition that the action of capillary force is effectively suppressed. Typically, carbon dioxide is used as the supercritical fluid, because it is excellent in handleability by virtue of the fact that it has a critical temperature of 31° C. and a critical pressure of 7.3 MPa, and is a noncombustible, nonpoisonous and inexpensive substance.
In this connection, it is recently reported that the pressure of carbon dioxide serving as the supercritical fluid is preferably 7.4 MPa.
In addition to the use in a drying process, the potential of applying supercritical carbon dioxide to a cleaning process for semiconductor wafers or the like draws high attention of researchers. While a wet processing is used in a conventional cleaning process, a serious problem has been turning up in connection with introduction of the structural minimization in the level of 0.1 μm or less, and new materials. That is, the conventional cleaning technique using liquid hardly allows the liquid to be fed into a microstructure, and thus an intended cleaning effect cannot be adequately obtained. Particularly in a new-generation insulation film having a porous structure for providing a lower dielectric constant thereto, a liquid introduced in micropores during a wet processing cannot be fully removed therefrom, and the remaining liquid would have a serious adverse effect on characteristics of a device to be obtained. In contrast, a cleaning process using supercritical carbon dioxide (supercritical fluid) having an excellent permeability close to that of gas allows the supercritical carbon dioxide to be readily infiltrated into a microstructure, without any problem of residual liquid in the microstructure as in the wet processing (see, for example, Japanese Patent Laid-Open Publication H09-232266).